Transmission line for radio frequencies



June 29, 1948. M. c. PEASE TRANSMISSION LINE FOR RADIO FREQUENCIES v INVEN TOR.

MARSHALL C. PEASE A T TORNE Y Patented June 29, 1948 TRANSMISSION LINEFOR RADIO FREQUENCIES Marshall C. Pease, Boston, United States of AmeriSecretary of War Mass, assignor to the ca as represented by theApplication September 14, 1945, Serial No. 616,386

2 Claims.

This invention relates generally to the transmission of radio frequencyenergy. More particularly its relates to low-loss radio frequencytransmission lines.

In many installations of apparatus operating at radio frequencies it isdesirable to transfer radio frequency energy over considerable distancesby means of coaxial transmission lines.

Where these lines are of appreciable length it is necessary to providesupports to keep the innerconductor properly spaced within the outerconductor. Numerous ways of doing this have been proposed and used inthe prior art but difficulties of one sort or another have beenencountered in their use.

The main disadvantage is the introduction of energy losses of suchmagnitude that the operation of the apparatus is impaired. At relativelylow radio frequencies, appreciable losses may be tolerated but at higherradio frequencies where the amount of energy available at the input endis limited and must be conserved if enough is to remain to accomplish auseful function at the output end, even a relatively small less becomescritical.

Resonant coaxial stubs of conductive material used as insulators havevery low loss at the frequency for which they are designed and thusserve efficiently at that frequency and over a very narrow band aboutthat frequency. However, the resulting line is of restricted usefulnessbecause once the designed frequency is departed from, energy losses riserapidly. The result is that such a line cannot be used for broad bandoperation. For broad band operation the art in general has used spaceddielectric discs for supporting the inner conductor. These have anadvantage over the use of a solid dielectric as a support in that thevolume of dielectric is reduced but they have a disadvantage because thecharacteristic impedance of the line changes at every point where asupporting disc is located. Thus, energy moving through such a line.encounters a discontinuity every time it passes from an air insulatedsection to a supporting disc and vice versa. This causes standing wavesand the losses, even if the line is otherwise a low-loss line, aresubstantially increased. Hence a great part of the advantage of reducingthe volume of dielectric gained in the spaced support construction islost.

In striving to produce low-loss coaxial transmission lines, the art hasbeen governed by the known fact that in general losses vary directly asthe volume of dielectric material between conductors. That is to say,these methods have been based on keeping the volume of dielectricmaterial between conductors at a minmum. It has been discovered that,although the above relationship is true, it is not the onlydeterminative factor in reducing losses and that attainment of theabsolute minimum volume of dielectric possible may be sacrificed inorder to construct a line having no discontinuities whatsoever.

It is an object of the present invention to construct a supportedcoaxial transmission line having a minimum amount of attenuation of theenergy passing through it in which the dielectric supports, while not ofminimum overall volume per length of line, are so designed that noelectrical discontinuity is present and the overall energy loss is at aminimum.

It is also an object of this invention to construct a coaxialtransmission line that may be used over a very wide :band of operatingfrequencies.

Generally this invention comprises a coaxial transmission line havinglongitudinally continuous dielectric supports occupying only a smallpercentage of the volume of space between the inner and outerconductors.

Other objects, features and advantages of this invention will suggestthemselves to those skilled in the art and will become apparent from thefollowing description of the invention taken in connection with theaccompanying drawings in which:

Fig. 1 is a sectional View of a bodying this invention; and

Fig. 2 is a longitudinal sectional view along line 22 of Fig. 1, showingan end section of such a coaxial line emline.

Referring now more particularly to Fig. 1, concentric transmission linei I) has an outer conductor II and an inner conductor I2 which areconcentional parts of such a line. Inn-er conductor I2 is formed so asto have three longitudinal keyways I3, I3, I3, running along its fulllength. Each of three dielectric supporting fins I 4, I4, I4 is ofproper width tp reach from the bottom of one of the keyways I3 to theinner surface of outer conductor II. Thus, when fully inserted intokeyways I3, I3, I3, dielectric fins I4, I4, l4 support inner conductorI2 in the center of outer conductor II and prevent it from moving to anyother position. Dielectric fins I4, I4, H- are of uniform thickness andrun for the full length of transmission line I0.

At the end of the line the radial width of dielectric fins I4, I4, I 4may be decreased, as shown at I 4a, Ida, Na, in Fig. 2, in a taperedmanner until there is no portion above the respective keyways l3, l3,[3. The purpose of this is to provide a gradual change in impedance butthe length of the tapered sections Ha, Ma, Ma of fins l4, l4, I4 shouldnot be so long that inner conductor l2 lacks necessary support.

The novel principles of design of a coaxial line employing thisinvention result in several desirable features. As may be seen byreferring to Fig. 1, the volume of dielectric material in the spacebetween outcr conductor II and inner conductor I2 is but a smallpercentage of the total volume of space between said conductors andtherefore the losses which would result from a solid dielectric supportare greatly reduced. Although the volume of dielectric in a coaxial lineemploying this invention may be no less than that of a spaced supportcoaxial line, the losses are less for the reasons stated below. That is,all elements of the line are continuous from one end to the other sothat any portion of the line is identical to all other portions and itscharacteristic impedance is uniform throughout its length.

The mechanical support provided in a line employing this invention isextremely rigid because the support is continuous for the whole lengthof the line thus avoiding any possible variation in characteristicimpedance.

It will be obvious to those skilled in the art that alternate details ofconstruction employing the principles of this invention may be used. Forexample, the number of dielectric fins may be increased or decreased orthe shape of the dielectric fins may be changed so long as they arelongitudinally continuous and uniform, occupy a relatively small volume,and provide adequate support.

It will be seen that the transmission line herein set forth willtransfer radio frequency energy with low-loss, cause minimum reflectionsdue to discontinuities within the line itself, and function over a verybroad band of operating frequencies.

While there has been described what is at present considered to be thepreferred embodiment of this invention it will be obvious to thoseskilled in the art that various changes and modifications 4 i may bemade therein without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:

1. In a coaxial line for transmitting energy at radio frequencies andhaving an inner conductor and an outer conductor, means for supportingsaid inner conductor within said outer conductor comprisinglongitudinally continuous dielectric fins radially fixed in, andparallel to, said inner conductor and having a portion at the end ofsaid line of gradually decreasing radial width, said fins occupying arelatively small portion of the space between said inner conductor andsaid outer conductor.

2, A coaxial transmission line for radio frequencies comprising an outerconductor, an inner conductor coaxial therewith, the surface of saidinner conductor being provided with a plurality of grooves formedparallel to the axis of said inner conductor and equiangularly disposedabout said surface, a plurality of longitudinally continuous dielectricspacing fins disposed between said inner and outer conductors, one edgeof each of said fins being fixed in a corresponding groove, said finsbeing thin relative to the cross sectional area of said coaxialtransmission line, the terminal portions of said fins being radiallytapered for gradually decreasing the radial width thereof whereby theterminal impedance of said line may be altered in accordance with saidtaper.

MARSHALL C. PEASE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,035,274 Mougey Mar. 24, 19362,149,223 Mason Feb. 28, 1939 2,396,871 Meyerhans Mar. 19, 1946 FOREIGNPATENTS Number Country Date Great Britain (1911) July 18, 1912

